This invention relates to electronic musical instruments and more particularly to such instruments of the digital synthesizer type in which the output waveform is computed from a plurality of component waveforms.
For purposes of this application, the following terms shall have the following meanings unless otherwise noted:
VOICE: a complex waveform consisting of pitched and unpitched components and including its amplitude envelope.
ADSR: attack-decay-sustain-release--the envelope amplitude waveform containing the voice.
TIMBRE: the portion of a voice consisting of pitched components only. Sometimes this term is used to include unpitched components also. In no case does it include the amplitude envelope.
PITCH: a term used synonymously with frequency.
FUNDAMENTAL: the pitched component with the lowest pitch. Note that the amplitude of a fundamental can be zero and still be valid. If sufficient overtones exist to establish the fundamental, it will be apparent even if it does not exist.
OVERTONE: a pitched component of higher frequency than the fundamental. Harmonics are a subset of overtones. The fundamental is not an overtone.
HARMONIC: a pitched component of higher frequency than the fundamental which also has an integral relationship in pitch with the fundamental. Since it is the reference by which harmonics are defined, the fundamental is not strictly a harmonic; yet, when included with harmonics as a set the fundamental is often referred to as the first harmonic.
PARTIAL: any pitched or unpitched component part of a voice.
FRACTIONAL: a new term connoting an overtone with a simple fractional relationship with the fundamental such as 1/2, 1/3, 1/4, etc.
It is well-known that pleasing musical sounds can ee generated electronically by building a complex pitched waveform as the sum of a fundamental wave (a fundamental pitch sine waveform) and selected harmonic overtones (sine waveforms with frequencies which are integral multiples of the frequencies of the fundamental). An electronic musical instrument of this type is disclosed, for example, in U.S. Pat. No. 3,809,786 to Deutsch.
Timbres or musical sounds produced in this way typically have complex waveforms which are fixed in shape from cycle to cycle. An advantage resulting from this characteristic is that a single cycle of the waveform may be placed in a memory and played back by traversing the memory at a desired fundamental pitch rate. An electronic musical instrument of this type is disclosed, for example, in U.S. Pat. No. 4,601,229 to Deutsch.
Although many timbre variations may be produced in this type system by varying the amplitude coefficients, the sound produced in this way is considered bland and uninteresting by listeners. In part this is due to the fact that sounds generated by mechanical musical instruments differ significantly from the sound generated by this type of digital system. For example, mechanical musical instruments generate overtones which have small variations inpitch from a true harmonic (i.e., integral) relationship with the fundamental. That is, the overtones of mechanical musical instruments are detuned somewhat. Since music listeners have become accustomed to such variations, they strongly prefer musical instruments that exhibit such overtone detuning.
Due to the constantly shifting phase relationships between the components of the complex waveform, pitched complex waveforms with detuned overtones are not uniform in shape from cycle to cycle. Consequently, they may not be placed in a memory for traversing playback unless a very large memory, capable of containing several seconds of music, is used. Such memories would be prohibitive in size, especially if a multiplicity of voices are to be contained for immediate playback. Therefore, a voice with detuned overtones must be generated in real time.
Several schemes have been devised for obtaining detuned overtones for real time playback. A scheme with limited abilities and very complex hardware requirements is shown in U.S. Pat. No. 4,513,651 to Deutsch This scheme provides three sets of partial amplitude coefficients (with consequent hardware triplication), which are mutually exclusive with respect to zero amplitudes. Each set of partial amplitude coefficients are utilized with a pitch slightly different from the other, summed to form the composite waveform. The three sets provide deviation for sets of partials, but do not allow individual pitch adjustment.
Another scheme, elegant in concept but overly complex in hardware requirements, is disclosed in U.S. Pat. No. 4,215,614 to Chibana. This scheme provides detuning coefficients and pitch coefficients in a logarithmic scale. When summed these coefficients are processed through a logarithmic to linear converter to provide a good substitute for the multiplication operation otherwise required. This patent also suggests that of the pitch variations such as vibrato, portamento and glide could be incorporated. This would be uneconomical, however, since these functions are quite slow in nature and can be handled more flexibly and cheaply by software in a controlling processor.
In addition to the overtone detuning mentioned above, mechanical musical instruments exhibit changes in overtone structure during the course of a single note. Some of these changes are associate with envelope amplitude and others with time. The overtone structure for the guitar, for example, is quite different at the half-amplitude point from that at its peak amplitude. These changes in overtone structure occur in both the overtone amplitude and overtone pitch.
Mechanical musical instruments also exhibit changes in overtone structure as a result of playing style. For example, the overtone structure of the clarinet played softly is not the same as the overtone structure when it is played loudly.
Modern music listeners have become accustomed to musical instruments which exhibit these overtone structure changes. Contemporary guitar musicians use many devices to adjust voice timbre during a rendition. And modern keyboard devices exhibit many timbre adjustment facilities.
another challenge which faces the designer of electronic musical instruments is that t would be advantageous if a musical instrument could contain a multiplicity of voices such that an instantaneous selection could be made by switch, either by exclusive selection or by sequential selection.
An effort to meet a small portion of these challenges has been proposed by Yoichi Nagashima et al in U.S. Pat. No. 4,612,838, issued Sept. 23, 1986. In that patent he proposes developing two voice structures, with differing amplitude coefficients, and interpolating between the two in order to obtain voices with intermediate overtone structure. The hardware complexity and computation time required for this interpolation scheme detracts from its usefulness. The limitations imposed by having only two sets of coefficients (even though intermediate points serve as other sets) is also undesirable. The Nagashima invention covers only overtone amplitude adjustments and does not address the overtone pitch adjustment problem.
Heretofore, designers of electronic musical instruments have been trying to "catch up with nature," i.e., trying to produce voices synthetically which have the listening appeal of mechanical musical instruments. Indeed, much effort has been expended in attempting to duplicate the sound of existing mechanical musical instruments.
This narrow approach to music synthesis has resulted in a dedication to producing voices composed of a fundamental plus integer or near integer related overtones (in which the overtones are full multiples or nearful multiples of the fundamental). It has been discovered, however, that in actual fact very pleasant voices may be constructed from a fundamental plus overtones that are multiples of one-half (or nearly so) of the fundamental. Such voices do not appear in any mechanical musical instrument. Other "fractional harmonics" such as one-third, one-fourth, etc. may be used to either construct voices or to augment the tonal qualities of more conventionally constructed voices, but the prior at appears to be unaware of this fact.
Another area of voice augmentation that has been overlooked in prior art electronic musical instruments is in providing a capability for a multiplicity of a particular partial. It has been assumed that since only one of each of a given set of partials appears in mechanical musical instruments, then only one is needed. It has been discovered, however, that a particularly striking augmentation of a voice may be realized by selecting one of the component partials and triplicating it, tuning one to the pure pitch ratio while tuning another flat and another sharp. The result (with small amounts of such detuning) is a singular voice with a choral overtone. An endless number of variations to this theme are available; for example, more than one partial may be so constructed with the second set tuned various degrees of sharp or flat, perhaps another set may be developed, detuned in the opposite direction. All of these vaiations, if properly designed, are interesting, unique and melodic voices with musical characteristics not available in any mechanical device.
Another defect in current schemes for voice construction arises from hardware that produces a component for the voice whether needed or not, even if it has a zero amplitude coefficient. In these prior art schemes, each possible component to be used in voice construction is assigned a time slot during computation and that time slot is used for the computation of that voice element, whether that component is present or not. A much more flexible scheme, disclosed in the present invention, would allow independent component assignment to the time slots.
Most voices have zero amplitude coefficients for some (or most) of the available partial components. If these time slots were free for reassignment, then the voices could be further augmented with multiple overtones or selected fractional partials. In fact, if a voice required only a few partials, the entire voice could be replicated. If the replicated voice were tuned properly, this would result in a choral effect, without the complication of adding further hardware.